Neurotransmitters, substances in the brain and nervous system responsible for carrying messages from one neuron to another,

Since the discovery of chemical synaptic transmission, the search has been on to identify neurotransmitters in the brain. Our current understanding is that most neurotransmitters fall into one of three chemical categories:

1. Amino acids

2. Aminas

3. Peptides

the amino acids and amina are all samll organic molecules containing a nitrogen atom, and they are stored in and released from synaptic vesicles.

Peptides are large molecules stored in and released from secretory granules. Secretory granules and synaptic vesicles are frequently observed in the same axon terminal. Consistent with this observation, peptides often exist in the same axon terminals that contain amino acid or amina neurotransmitters and these different neurotransmitters are released under different conditions.

Acetylcholine, compound in the body of a vertebrate animal that functions as a neurotransmitter, conducting electrical impulses across synapses between nerve cells, and from nerve cells to muscle cells, causing the muscle cells to contract. The effect of acetylcholine is then neutralized by an enzyme, such as cholinesterase. See Brain; Neurophysiology.

Serotonin, or lack of it, has been implicated not only in depression, anxiety and panic, but also in uncontrolable appetite and obsessive-compulsive disorder, autism, bulimia, social phobias, premenstrual syndrome, migraine, schizophrenia and even extreme violence.

The most common excitatory neurotransmitter in the brain is glutamate, a chemical that activates several subtypes of receptors. These occurs in large numbers in the hippocampus, part of the brain involved in memory and learning. Among them are NMDA (N-methyl-D-aspartate) receptors, which, like other such receptors, need both glutamate and an agonist (a chemical which "helps" to generate local postsynaptic potentials) - in this case glycine.

Without serotonin, dopamine, norepinephrine and the hundreds of other known neurotransmitters, the brain could not process information or send ou instructions to run the rest of the body. That is because neurons do not actually touch one another; they are separated by gaps known as synapses. When the electrical impulses that carry information through the nervous system reach the end of a neuron, they have nowwhere to go. The circuit is broken.